1. Field of the Invention
The present invention relates to rotor assemblies for electric machines.
2. Description of the Related Art
In the design of alternators, an important concern is the noise generated by the alternator when it operates. One way this concern is addressed in some "claw pole" type alternators is through the insertion of a plastic silencer in the rotor assembly of the alternator. This silencer fills the gaps between the fingers of the pole pieces of the alternator, causing the rotor's outer surface to be relatively smooth. Windage noise generated by the rotor when it rotates is thus reduced.
Such a use of a silencer introduces a concern, however. Rotors in alternators rotate at very high speeds (some in the vicinity of 20,000 revolutions per minute or more). The centrifugal forces developed at such speeds are very substantial and tend to cause radial expansion of the silencer. Such tendency toward radial expansion is compounded by the high underhood temperatures of many motor vehicles. If the radial expansion is not limited, it can cause the silencer to rub against the stator of the alternator. Failure of the alternator can result.
One potential way to prevent such expansion is illustrated in U.S. Pat. No. 4,617,485, issued to Nakamura. This patent discloses in FIGS. 3 and 4 a silencer with portions 3-b which extend under the pole fingers of the pole pieces of the rotors. However, such a configuration can have problems of its own. The configuration in that patent has a large amount of contact area between the plurality of pole fingers of the rotor and the silencer. Such a large contact area can create a significant frictional force to be overcome in pressing the two pole pieces together to assemble the rotor. Further, such a silencer configuration makes getting varnish to the interior of the assembled rotor a difficult proposition. (Varnish is often applied to a rotor to improve its structural integrity.)
Therefore, a silencer assembly which helps reduce rotor noise, does not present significant obstacles to rotor assembly and which facilitates getting varnish to the interior of the rotor will provide advantages over existing designs.